The present invention relates to a preform material applied to a precision press molding, and to an optical glass for precision press molding, which is suitable for a precision press molding with the preform material, and to an optical element.
Recently, optical devices are remarkably miniaturized and performance thereof is becoming remarkably higher. Thus, optical elements, such as lenses or the like, which are used for the optical devices, are also required to lighten, miniaturize and have high performance. Uses of aspherical lenses can decrease the number of lenses to be used and, for this reason, the uses of aspherical lenses are becoming the mainstream in designs of various kinds of optical devices.
As a producing method for the aspherical lenses, a so-called precision press molding is becoming the mainstream. In the precision press molding, a preform material obtained from a gob or a glass block is heated to soften, and then press-formed in a mold having a precise surface. The method can omit a grinding or polishing process for a lens after forming, which is a character of the method. Thereby the optical elements, such as lenses or the like, can be mass-produced at a low cost.
In order to achieve the object of the mass production at a low cost in the producing method by the precision press molding, a mold used for the precision press molding is desired to be capable of repeated uses. For that purpose, a temperature during the precision press molding should be as low as possible so that oxidation of the surface of the mold will be suppressed as little as possible by using a glass having a low-temperature softening property, that is, a glass having a low glass transformation point (Tg).
At present, the upper limit temperature, which is determined by a heat-resistance temperature of the mold, for the precision press molding is from 650 to 700xc2x0 C. Accordingly, the upper limit of the glass transformation point (Tg) is about from 550 to 600xc2x0 C., however, it is preferable to be as low as possible for suppressing progress of the surface oxidation of the mold and lengthening a life of the mold.
On the other hand, the glass that is used for the aspherical lens of the optical device is required to have various optical constants. Particularly, the glass having optical constants of refractive index (nd) of about from 1.65 to 1.73 and Abbe number (xcexdd) of about from 50 to 60 is required. Conventionally, as a glass having such optical constants, a lanthanum borate system composition has been typically known. For example, various glasses are disclosed in Japanese Patent Application Laid-open No. Sho 48-61517, Japanese Patent Application Laid-open No. Sho 54-3115 or the like. However, these glasses generally have high glass transformation point, so that these are unsuitable for a glass for precision press molding.
As for a producing method for the preform material, a method in which a glass melt is dropped from a tip of an efflux pipe, the dropped glass is received and formed in a mold or the like, and then it is cooled for obtaining the preform material has been known. Typical examples for obtaining the preform by this method are disclosed in Japanese Patent Application Laid-open No. Hei 6-122526, Japanese Patent Application Laid-open No. Hei 8-319124 or the like.
According to this method, mass-productivity is high and a production cost is the lowest at present. Furthermore, the preform material obtained by the method has a spherical or biconvex lens-like shape near a final shape of the lens. Thus, variations in shape of the preform material can be reduced during the precision press molding, so that the method has an advantage that a mass-productivity of lenses themselves, which will be finally obtained, can be improved.
On the other hand, another method for obtaining the preform material by cutting a glass block material has been known. However, after the block material is cut, this method requires a process for the cut block material to have a ball shape. If the process is not carried out, when the block material is formed from a rectangular solid to the lens shape, variations in shape increase, and then it takes a time for forming. Accordingly, the former forming method, in which the glass melt is dropped, is remarkably superior in the cost and the mass-productivity.
With the forming method, in which the glass melt is dropped, if the viscosity of the glass melt is too low, it is difficult to obtain the preform material having a smooth curved surface and the shape near the spherical or biconvex lens-like.
On the other hand, if the viscosity of the glass melt is too high, control for a dropping state of the glass melt, which is continuously flowed from the tip of the efflux pipe, becomes difficult. Thus, separating of only an amount required for one forming becomes also difficult, so that the producing of the preform material becomes difficult.
In addition, the glass is required to have a devitrification temperature that is lower than a temperature of the glass melt during the producing of the preform material. That is, if the viscosity of the glass melt is lower than the desired value, the lowering of temperature is required for increasing the viscosity of the glass melt because of the above-described reason. If the devitrification temperature is higher than the glass melt temperature at which the desired viscosity is obtained, lowering of the glass melt temperature for obtaining the desired viscosity causes occurrence of crystallite in the glass melt. As a result, devitrification occurs in the preform material, so that the preform material may not be used for the optical glass. Particularly, such a tendency becomes significant with the glass having a low viscosity.
Explaining more concretely, if the devitrification temperature is high, the devitrification occurs in the glass melt at the tip of the efflux pipe at a temperature at which the viscosity becomes such as to enable the forming of the preform material by dropping. As a result, the devitrification is contained on a surface or inside of the preform material, thus a yield of the preform material reduces extremely. For avoiding this, every constant time (about one to three hours), the temperature of the tip of the efflux pipe should be increased to a temperature at which the devitrification disappears. Therefore, continuous and stable production of the preform materials, which have high qualities, by the dropping method is extremely difficult.
Japanese Patent Application Laid-open No. Sho 60-221338 discloses a lanthanum borate system optical glass having a low glass transformation point, while Japanese Patent Application Laid-open No. Hei 5-58669 discloses a lanthanum borate system optical glass suitable for precision press molding. These glasses, however, have a devitrification temperature that is higher than a temperature at which the viscosity suitable for the production of the preform materials is obtained. Thus, the problem as described above arises.
On the other hand, when the viscosity of the glass melt is higher than the desired value for the forming of the preform, the temperature of the glass melt is increased for decreasing the viscosity, so that the problem for the devitrification does not arise. However, a problem of seizing of the glass melt in the mold for the preform materials arises, or a problem of early exhaustion of the mold by the surface oxidation of the mold arises. According to experiments, when the preform materials are formed not more than 950xc2x0 C., these problems do not arise.
That is, in addition to the low glass transformation point as previously described, the optical glass for precision press molding is preferable to have a devitrification temperature that is lower than the temperature of the glass melt, at which the viscosity suitable for the forming of the preform materials is obtained. Furthermore, it is preferable that the temperature of the glass melt, at which the viscosity suitable for the forming is obtained, is as low as possible in view of a heat-resistant temperature of the mold, which is used for forming of the preform.
In addition, a glass including PbO component is liable to fuse with the mold during the precision press molding. Therefore, repeated uses of the mold are difficult. Thus, such a glass is unsuitable for the optical glass for precision press molding.
Furthermore, a glass including F2 component causes clouds in the preform material because the F2 component is selectively volatilized from a surface layer of the glass melt during the forming of the preform material. With the glass including F2 component, the F2 component volatilizes to deposit on the mold during the precision press molding of the preform material, generating clouds on the surface of the mold. From such reasons, the glass including F2 component is unsuitable for the optical glass for precision press molding.
An object of the present invention is, by solving the problems of the conventional glass, to provide an optical glass for precision press molding that has optical constants of refractive index (nd) within a range from 1.65 to 1.73 and Abbe number (xcexdd) within a range from 50 to 60, and a low-temperature softening property, that is, a low glass transformation point (Tg), and that is excellently resistant to devitrification property and has a devitrification temperature that is lower than a temperature of glass melt, at which a viscosity suitable for forming of a preform material is obtained. Another object of the present invention is to provide an optical glass for precision press molding, which is free of PbO component and F2 component besides having above-described properties. Further object of the present invention is to provide an optical element.
In order to accomplish the objects, the inventors repeatedly carried out experiments of forming of a preform material by dropping a glass melt of lanthanum borosilicate system glass. As a result, the inventors found that a viscosity xcex7 (poise) range of the glass melt, which is suitable for the forming of preform material by dropping, is a range of log xcex7=from 1.5 to 2.5.
In addition, in order to obtain a stable glass that has no devitrification within the above-described viscosity xcex7 (poise) range of the glass melt, Y2O3 was tried to coexist with the composition system having SrO component and ZrO2 and/or Ta2O5 component as disclosed by the Japanese Patent Application Laid-open No. Hei 5-58669. However, unexpectedly, a stable glass was not obtained. With a composition system having Y2O3 component, it was found that additionally coexisting of CaO with the Y2O3 component improves the devitrification temperature not less than 100xc2x0 C. compared with the conventional glass. That is, it was found that within a very limited specific composition range of SiO2xe2x80x94B2O3xe2x80x94La2O3xe2x80x94Y2O3xe2x80x94Ta2O5xe2x80x94CaOxe2x80x94Li2O system composition, an optical glass is obtained optical constants within the desired ranges and a low glass transformation point, and is much excellently resistant to devitrification property, and that is synthetically superior in some properties required for the production of the preform material and the precision press molding. Thus, the inventors have achieved the present invention.
According to one aspect of the present invention, an optical glass for precision press molding according to the present invention comprises a lanthanum borosilicate system glass having optical constants of refractive index (nd) within a range from 1.65 to 1.73 and Abbe number (xcexdd) within a range from 50 to 60; and a glass transformation point (Tg) within a range of 480-580xc2x0 C. The optical glass for precision press molding has a devitrification temperature that is lower than a temperature of a glass melt when the glass melt has a viscosity xcex7 (poise) of log xcex7=2.5. The devitrification temperature means that a temperature at which crystallite occurs when the temperature of the glass melt, which is in a transparent state, is reduced.
The optical glass for precision press molding may comprise the following composition in weight percent of:
The optical glass for precision press molding may comprise the following composition in weight percent of:
According to another aspect of the present invention, an optical element according to the present invention is obtained by precision press molding an optical glass that comprises a lanthanum borosilicate system glass having optical constants of refractive index (nd) within a range from 1.65 to 1.73 and Abbe number (xcexdd) within a range from 50 to 60; and a glass transformation point (Tg) within a range of 480-580xc2x0 C. The optical glass has a devitrification temperature that is lower than a temperature of a glass melt when the glass melt has a viscosity xcex7 (poise) of log xcex7=2.5. The devitrification temperature means that a temperature at which crystallite occurs when the temperature of the glass melt, which is in a transparent state, is reduced.